1. Technical Field
The present invention relates to a rheological fluid which is responsive to a magnetic field.
2. Background Art
Rheological fluids responsive to magnetic fields are known. Such fluids, are referred to herein as electrorheological magnetic fluids (ERM) fluids and are used in clutches, shock absorbers, and other devices. A characteristic of these fluids is that, when they are exposed to a magnetic field, small solid ferromagnetic particles in the fluid move into alignment along lines of magnetic flux forming chains of magnetically connected particles. These chains have a significant strength in tension and hence substantially reduce the ability of the fluid to flow.
A clutch, shock absorber, or other such device for regulating the motion or energy transmitted between relatively displaceable members, will have spaced-apart relatively displaceable component parts. The ERM fluid is disposed between the parts. Decreasing the ability of the fluid to flow correspondingly increases the energy required to cause the component parts to move with respect to each other. This characteristic can be used to provide the desired clutch, shock absorber or other function.
ERM fluids may include a liquid carrier or vehicle and magnetizable solid particles suspended in the vehicle. The vehicle can be a dielectric medium, such as hydrocarbon, silacarbon, or silicone oil. Examples of magnetizable solid particles are iron, nickel, cobalt, magnetite, ferrite(s), Fe.sub.2 O.sub.3, alloys and compounds of iron, nickel, or cobalt with rare-earth elements, chromium, silicon, boron, mixtures of the above, and magnetizable stainless steel.
U.S. Pat. No. 4,992,190, to Shtarkman, assigned to the assignee of the present application, discloses a fluid responsive to a magnetic field. The fluid comprises iron powder, an oil vehicle, and silica-gel as a dispersant for the iron powder in the vehicle. Silica gel has an average particle size, typically, of about 0.05 micrometers (50 nanometers).
U.S. Pat. No. 2,772,761 to Janson discloses a composition for an electromagnetic clutch. The composition comprises iron particles, a vehicle, and graphite particles to prevent interlocking of the iron particles. The sizing of the graphite particles is imprecisely described as "colloidal".
U.S. Pat. No. 2,751,352 to Bondi discloses a magnetic fluid for a clutch or like apparatus which comprises an iron powder, a number of organic lubricants including polymers and silicones, and an oleophobic-ferrophilic liquid dispersant. The oleophobic-ferrophilic dispersant is described as a polar liquid capable of coating the iron particles to prevent them from being wetted by the organic lubricants.
U.S. Pat. No. 3,006,656 to Shaub discloses a magnetic material for shock absorbers which comprises carbonyl iron powder, and an additive such as oil, kerosene, benzene, graphite, chalk, mica, soapstone, a silicone, and glycerine. No particle sizings are specified in the patent.
A large number of patents disclose the use of dispersants for compounding ferrofluids. Ferrofluids are liquid/ferro particle mixtures of great scientific interest that have been used to provide fluid seals for rotating machinery, a coolant medium for a voice coil or loudspeaker, a dampening liquid in an inertia damper, a bearing liquid, or a ferro lubricant. U.S. Pat. No. 4,732,706 to Borduz et. al. discloses that stability is an important requirement in a ferrofluid and that the dispersants which are used are soluble and ionizable in the carrier liquid. Surfactant ions are preferentially absorbed from the solution onto the ferromagnetic particles. The ferromagnetic particles thus receive an electrostatic charge equal to the charge of the absorbed ions. This causes one ferromagnetic particle to be physically repelled away from another, in effect stabilizing the ferrofluid.
U.S. Pat. No. 4,957,644 to Price discloses the use of a ferrofluid in a clutch. The ferrofluid comprises magnetizable particles, a fluid carrier, and a chelating agent. The chelating agent dissolves in the carrier liquid. Molecules of the chelating agent bond to the magnetizable particles, and the magnetizable particles thus repel each other, as in the '706 patent.
U.S. Pat. No. 5,013,471 to Ogawa discloses a magnetic fluid comprising ferromagnetic particles, and an oil carrier. The ferromagnetic particles are dispersed by a chloro-silane type surfactant which forms an absorbed film on the surfaces of the ferromagnetic particles.
U.S. Pat. No. 4,280,918 to Homola discloses a magnetic dispersion containing magnetic particles and colloidal silica particles irreversibly bonded to the magnetic particles. The bonding of the colloidal silica particles to the magnetic particles is achieved electrostatically.
A publication entitled "Quest", dated Summer, 1986, on pages 53-63, contains an article "Exploring the TRW Carbons", by Jack L. Blumenthal et. al. The publication is published by the assignee of the present application. The publication discloses a particulate suspension for a fluidic device comprising iron powder (75% by weight), mineral oil, and 1% TRW carbon. TRW carbon is a filamentary carbon in which the carbon fibers have a diameter of about 0.05 to 0.5 microns and a length up to several thousand times the diameter. The particles are made in a catalytic carbon disproportion reaction in which a low heating value fuel gas or other source of carbon is used as the reaction feed.
U.S. Pat. No. 5,039,559 to Sang et. al. discloses magnetically attractable particles which comprise a core of magnetic material encapsulated in a metal oxide coating. Examples of the metal oxide coatings are aluminum oxide, silicon dioxide, titanium dioxide, zirconium oxide, and hydroxy-apatite (calcium phosphate). The particles are stated to have a diameter less than 100 microns.
Known ERM fluids exhibit a phenomenon known as "stick-slip" behavior. This behavior is characterized by a rapid non-linear increase in shear stress for relatively small deformations or motion up to a point and followed by a precipitous drop in shear stress with continued deformation or relative motion of the magnetizable particles in the ERM fluid. The shear stress level and drop off rate depend on several factors including the strength of the applied magnetic field. This behavior limits the allowable working stress that an ERM fluid will withstand and gives rise to undesirable fluctuations in applied shear stress. This can result in "chatter" if the load applied to the ERM fluid is near or above the point where the shear stress drops and the fluid cycles between its load-carrying phase and its slipping phase.